The present invention is directed to a sensor system designed primarily for geosynchronously orbiting spacecraft, although in slightly different configurations it can operate at all orbit altitudes. More specifically, the present invention is directed to improvements to attitude readout systems for geosynchronous orbiting spacecraft.
In order to successfully operate a spacecraft in geosynchronous orbit, the exact position of the spacecraft must be continuously maintained over a predesignated point on the earth, which is referred to as the spacecraft's allocated "slot". Through the use of an absolute clock, the spacecraft is able to precisely determine the location of the sun relative to the earth local vertical once the spacecraft is in its proper station. With this knowledge of the relative location of the sun, together with knowledge of the exact day/date of the year at four zones throughout the twenty-four hour orbit period, a two axis sun sensor has the capacity to provide three axis attitude information relative to the earth local vertical. However, a sun sensor cannot readout roll attitude at the spring and fall equinoxes when the sun is aligned with spacecraft roll axis (e.g., 9:00 a.m. and 3:00 p.m. local time), and the roll sensitivity significantly degrades at times of the year other than the equinoxes so that the sun sensor cannot readout the roll attitude near these local times. Similarly, the sun sensor cannot read out the yaw when the sun is at 12:00 noon local time. The sun sensor also cannot be used when the sun is behind the earth (21 days before and after both the spring and fall equinoxes) and other sensors must assume the attitude readout function.
A Polaris star sensor is based upon the same operating concept as that of a sun sensor, and uses the same detector and signal processing means as a sun sensor. Accordingly, a Polaris star sensor is a simple sensor in which Polaris recognition and acquisition is accomplished by the sun sensor. Accordingly, a Polaris star sensor can operate for a large part of the portion of the orbit of the spacecraft in which the sun is not behind the earth. During the sun-visible portion of the orbit, an infrared earth sensor is capable of providing a readout of the pitch attitude of the earth, and can hold the pitch attitude when the sun is behind the earth.
It is the primary object of the present invention to provide a sensor system designed primarily but not exclusively for geosynchronously orbiting spacecraft and having a three axis attitude readout system in which each of the sensors cooperates with other sensors in the system to emphasize advantages of each type of sensor and to compensate for inherent disadvantages in the other types of sensors employed in the overall system.
It is a further object of the present invention to provide a a sensor system primarily but not exclusively for geosynchronously orbiting spacecraft in which the overall sensor system is of low weight, low volume, low power consumption, and capable of being fabricated at a low cost.
Further objects and advantages of the invention will become apparent from the following description and discussion thereof.